This invention relates to methods of and circuits for suppressing clutter and noise for automatic detection radars. It is applicable to pulse doppler and MTI (moving target indication) radars, for which the principles are given in Chap. 4 of Introduction to Radar Systems, McGraw-Hill Book Company, 1980, by M. I. Skolnik. Throughout this document the term "doppler radar" refers to either MTI or pulse doppler radar.
Radar clutter signals are unwanted signals caused by radar echo (see, e.g., Skolnik, p. 470), and clutter signal magnitude therefore depends on radar range and direction. Land and sea clutter, being from stationary and slowly moving objects, have relatively small doppler frequencies including frequencies at and near zero. Noise signals, on the other hand, are unwanted signals caused by random fluctuations having wide doppler frequency spectra extending from zero to an upper limit controlled by receiver bandwidth (Skolnik, pp. 23-29). Unlike clutter echo, noise at the radar output occurs at all radar ranges.
Doppler radars are designed to detect radar echoes from moving targets having doppler frequencies appreciably above zero and to suppress signals having doppler frequencies at and near zero. For a stationary radar, the doppler frequency at the input of the doppler filter is zero for stationary clutter received from the earth's surface. Stationary clutter is then suppressed by the doppler processor comprised of a MTI canceler, bandpass filters having passbands not including zero frequency, or a combination thereof.
Because of the relative motion between the antenna and the earth's surface, most doppler radars for aircraft and ships employ motion compensation prior to doppler filtering so that surface clutter received via the antenna's major lobe (main beam) is suppressed. This is accomplished, e.g., by introducing an adaptive doppler frequency offset so that, at the input of the doppler filter, the signals from surface clutter received via the main beam have doppler frequencies at or near zero frequency. Clutter-lock and TACCAR (time-averaged clutter coherent airborne radar) are commonly used techniques to compensate for platform motion.
Radars have phase and amplitude instabilities that cause the signals they process to fluctuate, thereby creating modulation components of non-zero doppler frequency. Then, even when both the radar and the clutter are stationary, the doppler processor output due to said clutter, i.e., clutter residue, is oftentimes stronger than the output signals caused by moving targets of interest. Thus, strong clutter echoes received via an antenna's main beam, i.e., major lobe, are a source of main-beam clutter "burnthrough".
For a moving radar, surface clutter from the earth received via the antenna sidelobes constitutes another source of non-zero doppler frequency clutter. Recall that clutter-lock and TACCAR processing are designed to frequency shift received signals so that the doppler frequency of main beam surface clutter is at zero doppler frequency. However, on a moving platform the doppler frequency of clutter from a surface patch depends on the azimuth and elevation angles between the platform's velocity vector and said patch. Therefore, the doppler frequency after TACCAR and/or clutter-lock processing of surface clutter received via the antenna sidelobes is not zero, and consequently it is generally not suppressed by a doppler processor.
Rotating blades can also generate clutter signals at the output of a doppler processor because moving objects near a radar antenna will modulate the antenna's radiation pattern. Thus, rotating blades can modulate radar received signals, and the doppler frequencies generated are harmonically related to the rotation rate of the blades. Therefore, aircraft propellers and helicopter rotors can shift the doppler frequencies of zero and near zero doppler frequency clutter signals. Then, the clutter doppler frequencies, when shifted, can equal those of moving targets. In this case, the output of a doppler filter will contain clutter signals if a propeller modulation frequency component is within the filter pass band.
Modern automatic detection and tracking radars use an interference thresholding circuit called CFAR (constant false alarm ratio). A CFAR establishes a threshold level at each range cell to automatically reject clutter and noise. Then, a signal of magnitude above the threshold is assumed to be due to a target and one below the threshold is assumed to be caused by either noise or clutter. The most commonly used CFAR is the range CFAR. A range CFAR sets a threshold level in each range cell based on sampling the strength of a radar processed signal in neighboring range cells. To accomplish this, the CFAR obtains an aggregate (usually an average) of the strengths of the radar signal sampled at neighboring range cells, and based on the aggregate's magnitude it sets a threshold. Then for each range cell, a target signal is provided as output if its magnitude exceeds the threshold at said each range cell; otherwise the CFAR output is zero.
The present disclosure teaches radar detection in the presence of three types of clutter; herein called fixed, moving, and propeller clutter. For this document, fixed clutter is defined as clutter at the input to the doppler processor with doppler frequency at and near zero, and it is of course expected to be suppressed by the doppler processor. On the other hand, moving clutter has higher doppler frequencies at the input to the doppler processor and ordinarily it will not be suppressed by the doppler processor. Moving clutter includes both wind-driven rain and clutter from the earth's surface received via a sidelobe of a moving radar. Propeller clutter is caused by clutter signals from the earth's surface that have been shifted in doppler frequency by rotating blades, as described above, and it has doppler frequency equal to that of a moving target. Propeller clutter is therefore doppler frequency sidebands created by modulating surface clutter. Thus, propeller clutter is generated by the presence of radar received clutter of zero and near zero doppler frequency.
U.S. Pat. Nos. 4,459,592 and 4,684,950 teach a ratio comparator for establishing an adaptive clutter threshold level to reject clutter spikes, even though they are very strong. More specifically, a ratio comparator is taught that functions on the basis of the ratio of the amplitudes of two signals: one with magnitude proportional to the radar received signal and which contains doppler frequencies at and near zero and the other with and having doppler frequencies appreciably above zero. According to the definitions given above for fixed, moving, and propeller clutter; the ratio comparator rejects fixed and propeller clutter, but it does not reject moving clutter. The ratio comparator is designed, of course, so as not to reject signals from moving targets.
The subject invention comprises a ratio comparator (RC) and a level sensor (LS) means that function in operative association. Signals of intermediate amplitude or larger if due to fixed or propeller clutter are rejected by the RC-LS combination, Other signals including those from moving targets and moving clutter are not rejected, and are thereby provided as input to a range CFAR. In this way, if fixed or propeller clutter is present at the processor output, the average clutter level at the CFAR input is reduced. Thus the radar detection performance for moving targets is improved.